1. Field of the Invention
The present invention relates to a method of spectrographically measuring density of a strip of photographic negative color film with images recorded thereon and more particularly to a method of spectrographically measuring density of a strip of photographic negative color film which is suitably employable for exposure control of a photographic printing machine in which images recorded on a strip of photographic negative color film are printed on photographic printing paper.
2. Description of the Prior Art
As is well known to any expert in the art, a photographic printing machine (hereinafter referred to as printer) in which images on a strip of photographic negative color film obtained under various image taking (photographing) states or exposure conditions are printed on photographic printing paper (hereinafter referred to printing paper) is provided with control means which comprises an exposure control circuit and others in order to assure that the printer is operated under the optimum exposure conditions which are obtained by way of the steps of spectrographically measuring relative density, degree of saturation, average density or the like of the negative and then correctively determining exposure conditions for the printer in consideration of the results of measurement made in that way as well as photographic properties inherent to a printing paper to be used. Thus, printing operation is performed automatically by operating the printer.
Incidentally, the conventional method of spectrographically measuring density of a negative of the above-mentioned type is normally carried out by way of the steps of projecting an image recorded on a negative on light beam receiving faces of photoelectrical light beam receiving means with the aid of a plurality of projection optical systems including color filters for three primary colors and then making integration comparison with respect to output of each of the photoelectrical light beam receiving means.
However, it has been pointed out that a drawback with the conventional method of the above-mentioned type is that proper exposure conditions cannot be obtained with respect to a negative on which so-called density failure or color failure occurs (i.e., abnormal density or color distribution, such as too dark or too light) because only average value is obtainable over the whole image on a negative when the conventional method is employed for the intended purpose.
To obviate the foregoing drawback, a proposal has been made for improving the conventional method of spectrographically measuring density in such a manner that a negative having images recorded thereon is divided into a plurality of smaller divisional faces, the thus-divided faces are scanned and density is measured with respect to each of the smaller divisional faces whereby the optimum exposure conditions are obtained. However, the proposed method also has the drawback that a scanning portion becomes excessively large and complicated in structure due to the necessity for utilization of a light beam. This leads to a result that an apparatus for carrying out the method is expensive to manufacture.